Fiber optic (digital) communication systems are now preferred over and being installed to replace a variety of conventional cable network systems, primarily due to their wide spectral characteristics that allow a user to transmit broadband signals, as well as their flexibility in terms of the available choices for data rates. However, at very high data rates (for example, Gb/s or higher), the limited performance capabilities of readily available and reduced-cost electronic circuits and components has limited the end-to-end link distance of fiber optic networks to essentially that of a local area network, covering a distance of on the order of 5 km or less. As telecommunications customers are increasingly relying upon the rapid information access and transport capabilities of digital communication networks, it has become apparent to most service providers that the need exists to extend the range of high speed data communications to distances well beyond that of the local area network limit, but in a manner that is both transparent and cost acceptable to the end user.
In some of the network solutions, an optical transponder is used to extend the range of a full duplex fiber optical communication system upwards of 30 to 100 km. The fiber optic transponder includes a front-end (short haul) transceiver unit that contains an opto-electronic converter-receiver and an associated electro-optic converter transmitter. The front end's opto-electronic converter-receiver is coupled to an optical fiber of a local area network, through which gigabit digital data is supplied that is to be transported over a long distance fiber optic link for delivery to a recipient customer site. While the LAN fiber may be either multimode or single mode, the long distance fiber is required to be single mode, exhibiting a zero dispersion wavelength of either 1310 nm or 1550 nm, where the 1550 nm single mode fiber is primarily used in the prior art for the longest distance transmission systems. The electro-optic converter-transmitter unit is operative to convert electrical signals that have been regenerated from long distance optical data received from a far end site into optical signals for delivery to the LAN.
The optical transmitter included in the output of the transponder preferably includes a high speed, low jitter, current-limiting driver, which minimizes jitter generation, and thereby optimizes range extension margin. In most prior art transmitter arrangements, the current driver is controlled by a regulated drive current controller to ensure that the output extinction ratio of the laser diode is able of precise setting and remains highly stable, thereby minimizing wavelength chirp, so as to prevent undesirable dispersion effects through a dispersive, long fiber. To minimize potential dispersion for the long distance fiber link, the laser diode of choice in the prior art has been the distributed feedback (DFB), due to its narrow spectral width and an output wavelength that matches the zero dispersion wavelength of long haul transmission fiber (i.e., 1550 nm).
In some newer arrangements, 1550 nm electroabsorption modulated lasers (EMLs) are being deployed in high speed, 2.5 Gb/s and 10 Gb/s fiber optic networks. The advantage of these devices, as compared to the DFB lasers mentioned above, is that electroabsorption modulated lasers exhibit highly superior eye diagrams, with less pulse distortion/ringing, minimal chirp characteristics, high extinction ratio, and simplified driver circuitry. At the same time, there is a rapid increase in the deployment of fiber optic-based equipment which utilize transponder, transceiver and transmitter modules operating at 10 Gb/sec and at wavelengths near the 1310 nm dispersion minimum of optical fiber. Currently, directly modulated 1310 nm DFB or Fabry-Perot (FP) lasers are utilized in these applications. However, directly modulated DFB and FP lasers exhibit severe limitations due to relaxation oscillation effects and the difficulties of modulating the drive current at 10 Gb/sec. Thus, a need remains in the art for a laser source that is useful in the “intermediate” range (e.g., 10-50 km) between short haul (5 km) and long haul (over 100 km) applications, when using optic fiber with a zero dispersion wavelength at 1310 nm, that overcomes the drawbacks of the directly modulated DFB and FP lasers.